The present invention is directed to a communication method and system so as to provide for communication between a host communicator unit and one or more remote units in a noisy communication environment. The host communicator may be portable and interconnects with a plurality of remote units via a communication bus. These remote units may be emergency lighting or exit sign devices with self-testing capability as well as communication capability. Background concerning such emergency lighting units and exit signs having self-testing capability is disclosed in co-pending U.S. Application, Ser. No. 07/048,530, filed May 8, 1987, for an Emergency Lighting Supervisory System, assigned to the present assignee, and hereby incorporated by reference.
One of the problems encountered in a system designed to be interconnected with a plurality of remote units using a bus, wherein a plurality of such units are connected across the bus, is the requirement that communications be reliable and that they operate in environments where such emergency lighting and exit signs are located, typically in commercial and industrial establishments which may have high ambient electrical noise due to electrical motors, fluorescent lighting fixtures and the like.
The present invention addresses this problem by incorporating a data detection method which provides for the automatic adjustment of the receiver associated with the communicator so as to determine the remote unit's actual transmission rate which may vary widely from a nominal transmission rate. It is desirable to reliably detect such remote units since such units are significantly less expensive to manufacture if crystal controlled transmitters are not utilized.
The present invention detects the actual transmission rate from the remote unit by use of a start of message (SOM) header element from the remote unit which thereby allows for the susequent accurate detection of data "1" bits and data "0" bits received from the remote unit. The SOM also denotes the start of the entire message from the transmitting unit.
The present invention then, through its associated electronics, allows the user at the host communicator to interrogate one or more of remote units as to their operating status as well as to initiate self-testing operations.